1. Field of the Invention
The invention relates to the field of metallurgy and particularly to the field of titanium base alloys having excellent formability and method of making thereof and method of superplastic forming thereof.
2. Description of the Related Art
Titanium alloys are widely used as aerospace materials, e.g., in aeroplanes and rockets since the alloys possess tough mechanical properties and are comparatively light.
However the titanium alloys are difficult material to work. When finished products have a complicated shape, the yield in terms of weight of the product relative to that of the original material is low, which causes a significant increase in the production cost.
In case of the most widely used titanium alloy, which is Ti-6Al-4V alloy, when the forming temperature becomes below 800.degree. C., the resistance of deformation increases significantly, which leads to the generation of defects such as cracks.
To avoid the disadvantage of high production cost, a new technology called superplastic forming which utilizes superplastic phenomena, has been proposed.
Superplasticity is the phenomena in which materials under certain conditions, are elongated up to from several hundred to one thousand percent, in some case, over one thousand percent, without necking down.
One of the titanium alloys wherein the superplastic forming is performed is Ti-6Al-4V having the microstructure with the grain size of 5 to 10 micron meter.
However, even in case of the Ti-6Al-4V alloy, the temperature for superplastic forming ranges from 875.degree. to 950.degree. C., which shortens the life of working tools or necessitates costly tools. U.S. Pat. No. 4,299,626 discloses titanium alloys in which Fe, Ni, and Co are added to Ti-6Al-4V to improve superplastic properties having large superplastic elongation and small deformation resistance.
However even with the alloy described in U.S. Pat. No. 4,299,626, which is Ti-6Al-4V--Fe--Ni--Co alloy developed to lower the temperature of the superplastic deformation of Ti-6Al-4V alloy, the temperature can be lowered by only 50.degree. to 80.degree. C. compared with that for Ti-6Al-4V alloy, and the elongation obtained at such a temperature range is not sufficient.
Moreover, this alloy contains 6 wt. % Al as in Ti-6Al-4V alloy, which causes the hot workability in rolling or forging, being deteriorated.